Considerations in Contact Lens Use Under Adverse Conditions: Proceedings of a Symposium (1991)

Joshua E. Josephson

There are several occupational health concerns that may be directly or indirectly related to the environment of a military pilot.

Gauvreau's (1976) study of free-fall-jumping parachutists fitted with soft lenses revealed that corneal epithelial punctate staining and temporarily reduced visual acuity resulted when lenses were blown off the eye.

Sweat is an uncomfortable occupational problem. It is a concern for people who wear soft and hard lenes and for nonusers. Sweat dripping into the eyes can not only be uncomfortable but can also disturb the lens wearer's vision. A lens wearer can be distracted by excessive blinking or the desire to keep wiping their face or eyes with their hands. This may be a significant concern for the fighter-attack-reconnaissance aircraft pilot in combat. It may also be more of a long-term problem for the soft lens wearer, if the sweat is absorbed by the lens. Distraction and sensory interference have accounted for aircraft incidents (Billings and Reynard, 1984).

Extended wear may be an advantage if the pilot is on an operation or in time of war. A pilot must be ready to “scramble” and may have no time for lens insertion.

Zuccaro et al. (1985) reported a 3 percent incidence of neovascularization with prolonged extended wear after a 5-year clinical assessment. The criterion for neovascularization was vascularization greater than 1.5 millimeters. This does not realistically represent true neovascularization as substantiated by a recent U.S. Army study (Bachman et al., 1987) performed with soldiers wearing prolonged extended-wear lenses on field maneuvers. After 3 months, at the conclusion of the study, 29 percent of the spectacle wearers had neovascularization, 60 percent of the inexperienced contact lens wearers had neovascularization, and 63 percent of the experienced wearers had neovascularization. Any observed neovascularization was included in the data.

Since pilot activities during operations or time of war are usually of short duration (a maximum of 3 days), intermittent extended wear rather than prolonged wear may be advisable, as the risk of ocular complication would be reduced. Also, although increased oxygen transmissibility of soft lenses is thought to minimize neovascularization, other factors may play a role, such as trauma or inflammatory response to retained metabolic toxic waste or static exfoliated cells (Josephson et al., 1987). Increased lens motion may be helpful in preventing vascularization.

The risk of infection is greater in individuals who wear their lenses on an extended-wear basis rather than on a daily-wear basis. The occurrence of an infection can temporarily or permanently affect visual performance. The risk of a more severe outcome is greater in remote environments or during extended maneuvers. The occurrence of an infection, even temporarily, can put a pilot on the sidelines for anywhere from a day to several weeks, depending on the severity of the infection.

Smoke may be a related environmental hazard. Smoke can hypothetically decrease preocular tear film stability (Basu, 1977), and thus cause ocular irritation by microparticle contamination and disruption of the tear film.

The cockpit environment is highly contaminated with dust and particulate debris. Entrapment of a foreign body between the lens and eye will cause tearing and may be painful, possibly resulting in loss of control. Certainly, vision would be temporarily disrupted for at least 5 to 15 seconds, which could be a serious hazard if flying on instruments. The insult could

cause superficial corneal abrasions and perhaps a later complicating infection. In addition, the lens could be displaced off the cornea or even displaced from the eye because of excessive tearing or eye rubbing in response to the irritation. These occurrences could result in reduced vision, distraction, and, possibly, loss of control of the aircraft. It would follow that the lens could be difficult to remove if there is excessive tearing and blepharospasm. This would lead to further disruption of flight control.

Pilots who wear hard contact lenses have an increased risk of entrapment of particulates between the lens and the eye compared with soft lens wearers, because, as Fatt (1969) has shown, 15–20 percent of the prelens tear volume is exchanged with each blink, when a rigid lens fits properly. However, with soft contact lenses only 1–4 percent of tear volume between the lens and eye is exchanged with each blink (Wagner et al., 1980; Polse, 1979). Therefore, entrapment of particles between the lens and the eye is least likely for soft lens wearers, particularly those fitted with larger lens diameters. In addition, accidental lens displacement would be much less likely.

Nilsson et al. (1981, 1983) studied the effects of the mechanical trauma of airborne particles on rabbits fitted with hard and soft lenses. The animals were exposed to very hot grit particles. Although both hard and soft contact lenses protected the eye, the authors believed that industrial environments, heavily contaminated with airborne particles, are unsuitable for contact lens wear unless protection is used and the eye is fully sealed from the environment. However, prolonged sealing may result in eventual cornea hypoxia. Also, this study is an extreme condition of particulates in an immediate environment. It demonstrates that soft lenses can be worn in a dusty environment with little cornea risk. However, ocular discomfort may be expected with the accumulation of debris in the cul-de-sac and rubbing of debris by the lid over the bulbar conjunctiva.

Crosley et al. (1974) demonstrated that subjects with soft lenses were virtually free of foreign body entrapment under their lenses in dusty environments.

Unfortunately, there are very few scientifically controlled studies that explore the use of contact lenses in industrial settings. There are a number of anecdotal reports in the literature, but conclusions based on scientific studies, particularly studies on humans, are difficult to find. Therefore, it is difficult to appreciate the effects of chemical contaminants in the cockpit environment without further research being done.

The effects of chemical vapor or mist/droplet pollutants in the air could vary depending on the type of lens being worn. It is likely that these contaminants would affect a hydrogel lens, a nonpermeable rigid contact lens, a rigid gas-permeable (RGP) lens (some have lipophilic surfaces), or a

silicone elastomer differently. A rigid contact lens will not absorb chemical vapors or chemicals that contaminate it. It has been reported that rigid lenses will partially block a chemical splash from covering the ocular surface.

With the hydrogel lens wearer, chemical vapor can contaminate the lens and, if water soluble, can be absorbed. The effect of the contaminate would depend on the concentration, the amount of contaminate absorbed, the exposure time, the rate of elution of the sorbed chemical (the rate of release), the toxicity of the chemical, the tendency to allergic reaction to the chemical, and the severity of allergic response.

Oil mist/vapor contaminants in the air pose two risks. First, there can be reduced visual quality because of hydrophobic effects of oil on the lens surface. This would occur most readily with lipophilic surfaces, as are commonly found in silicone acrylate-base RGP materials. The second possible effect is the oil mist or vapor acting as an ocular irritant. The contaminant would disrupt the stability of the tear film and possibly irritate the ocular surface.

The use of contact lenses in environments with fumes from organic solvents or splashes of strong acids and alkalis was studied by Nilsson and Andersson (1982). They found that absorption of organic solvents, specifically trichloroethylene and xylene, by contact lens materials is not at all as dangerous as might be expected. They proposed that the contact lenses acted as a “vacuum cleaner” with the solvents. Therefore, the eyes were exposed to a lower concentration than if exposed directly without a lens in place. It should be noted that other solvents may react differently. Unfortunately, Nilsson and Andersson did not measure the concentration and rate of elution of the absorbed fumes. In addition, they did not consider the differences in possible ocular reaction with the four classes of hydrogel materials or consider lenses with various water contents. Their study was also a short-term one and did not demonstrate the effects of wearing contact lenses over a period of weeks or months.

Nilsson and Andersson also studied the effects of acids and alkalis on contact lenses worn on rabbit corneas. They considered both high-and low-water-content lenses in their study. They found that wearing soft contact lenses did not worsen the corneal damage caused by these chemicals. They evaluated the effects of a single drop of hydrochloric acid with a 40 percent concentration and one with a 20 percent concentration. They observed that soft contact lenses did not seem to worsen corneal damage and that with the acids in particular there was no significant damage even after 2 minutes of further exposure to the chemicals via the lens.

Guthrie and Seitz (1975) simulated chemical accidents to rigid contact lens wearers by instillation of 0.1 milliliters of 5 percent acetic acid, 0.5 percent butylamine, and 50 percent acetone into the eyes of rabbits. The left eye was fitted with a hard contact lens, and the right eye served as a

control. The eye with the contact lens provided more protection for liquid irritant exposures than the nonprotected eye. Guthrie and Seitz believed that lid spasm caused the lens to tighten around the cornea, sealing it off under the contact lens and thus preventing further serious injury to that section of the ocular surface.

Rengstorff and Black (1974) viewed 128 documented instances of chemical irritation and physical trauma to contact lens wearers. They observed that damage often occurred to the contact lens itself, with minimal or no injury to the cornea. The authors concluded that rigid contact lenses minimized injury or protected the eyes from more serious injury.

Pitts and Lattimore (1987) describe hexanes as virtually insoluble in water and having a low affinity for the plastic material of hydrogel contact lenses. Pitts and Lattimore reported that ethyl acetate is soluble in water and therefore could be taken up by the lenses. The effect of ethyl acetate is an insidious and chronic conjunctivitis with a superficial keratitis buildup over days of exposure. The victim would experience extremely irritated eyes, and, if the fumes were also inhaled, there would be a similar response in the mucosa of the nose, throat, and even the lungs. No effect has been reported on the peripheral or central nervous systems. However, Nilsson and Andersson (1982) demonstrated that the uptake of certain solvents by hydrogel contact lenses did not exacerbate the ocular response to the chemicals compared with direct exposure of eye and that in some situations the eye was protected.

Ozone has been reported by Daubs (1957) and Van Huesden and Mans (1978) to be increased in the environment of commercial aircraft. Daubs reported ocular discomfort in both lens wearers and nonwearers when increased ozone was present. The short- and long-term ocular effects of increased ozone with soft and hard contact lens wearers are not known.

Although there have been reports that soft and hard lenses protect the eyes from chemical burns, when there is ocular irritation the difficulties with immediate removal of a soft or hard lens can pose a serious problem that can further complicate the situation. With a hard lens, eye rubbing and blepharospasm may create a situation that can make the lens very difficult to remove. Eye rubbing with hard lens wear grip could cause the lens to suction onto the ocular surface. With soft contact lenses, a change in tear tonicity can cause the lens to stick to the ocular surface, further complicating a difficult removal situation.

Another potentially significant problem may occur when an individual, without knowledge of it, is inadvertently exposed to chemical vapors or fumes. A similar risk may occur if an aware contact lens wearer is exposed to chemical vapors or fumes but does not suffer significant symptoms or remains symptom-free afterward. In these situations ocular changes may occur while the individual may continue to wear the lenses for weeks or

months before a problem is actually realized or until a lens requires replacement. Studies have been performed only on the short-term effects of immediate contamination and have not focused on the possibility of the effects of chemicals at low dosages retained by hydrogel lenses and slowly eluted to the ocular environment over a period of days, weeks, or months. Williams (1986) has reported incidents of “toxic occlusion phenomena,” and others have anecdotally related cases of contamination of hydrogel contact lenses without immediate obvious effects to the patient, which were thought to cause adverse ocular responses.

High G_z forces may affect the meridional orientation and position of toric lenses. It is noteworthy that a prism made by wedge construction stabilizes the lens by the effect of lid pressure, not weight. Therefore, the term ballast is inappropriate. Hanks (1983) demonstrated that toric lens orientation does not change if the contact lens wearer stands on his or her head. The prism is still aligned with the lower lid by upper lid pressure, so normal gravity has no effect on lens orientation.

Forgie (1981) simulated higher than normal gravity forces in experiments with spherical soft lens wearers. Subjects were fitted with 15.0-millimeter diameter, 13.0-millimeter optic zone diameter lenses. Twelve eyes were observed under 5 G_z (= 4.2 Ge) and six eyes under 6 G_z (= 5.1 Ge). A lens displacement of 0.3 to 0.8 millimeters upward occurred in four eyes due to lid tightness, squeezing, and blinking and when there was a loose fit. A downward displacement of 0.1 to 3.4 millimeters occurred in 14 eyes. In this experiment displacement was insufficient to leave the pupil uncovered by the optical zone of the lens. Forgie (1981) reported isolated incidents where up to 7 G_z produced no detectable loss in vision with lens slippage. However, with hard lenses there may be reason for concern for the effects of exposure on the peripheral cornea, lens adhesion after decentration or lens edge compression on the peripheral cornea or adjacent bulbar conjunctiva.

Brennan and Girvin (1985) found no significant effect of vibration on the visual performance of soft lens wearers compared with spectacle wearers.

The effects of a reduced oxygen environment were covered by Dr. O 'Neal at this meeting. However, not mentioned was the work of Fonn (1986) on

soft-lens-induced edema with three different thicknesses of polyhema lenses (38.6 percent) at high altitude.

Dry environments are frequently encountered in aircraft cabins and are known to produce ocular discomfort (Eng, 1979; Rohles, 1988), and in some soft lens wearers, a significant epithelial disruption (Holden et al., 1986; McNally et al., 1987; Orsborn and Zantos, 1988). This epithelial lesion is less frequently observed if relatively thick lenses are worn (> 0.12 millimeters). In addition, changes in lens performance may be associated with lens parameter changes that can occur if free water is lost from the hydrogel matrix (Andrasko and Schoessler, 1980). Under experimental conditions, low-humidity conditions (10 percent and 30 percent relative humidity) did not significantly affect visual acuity, refractive error, or corneal curvature in eyes wearing soft lenses and control eyes without contact lenses.

The pilot's environment may not relate directly to all of the risks described in these studies. However, these reports do provide indirect information on potential risk situations. Clearly, additional controlled studies must be performed before predictions can be made about the actual risks of contact lens wear in an aircraft cabin environment, particularly during wartime.

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